The present invention relates to a system for monitoring the status of a filter in a hydraulic circuit.
It is now possible, due to microprocessor technology, to monitor various vehicle operating parameters according to complex criteria. For example, a transmission oil pressure monitoring system is described in U.S. patent application Ser. No. 379,230 filed May 17, 1982, a clutch monitoring system is described in U.S. patent application Ser. No. 379,231 filed May 17, 1982, and an engine oil pressure monitoring system is described in U.S. patent application Ser. No. 378,704 filed May 17, 1982, all assigned to the assignee of the present invention. However, none of these monitoring systems provide an indication of the status of a filter in a hydraulic circuit, such as the hydraulic circuits of a hydraulically controlled and lubricated vehicle transmission.
It is known to monitor the condition of a fluid filter by means of a switch which is opened or closed, depending upon the condition of the filter, as described in U.S. Pat. No. 4,216,524. Another filter monitoring system described in U.S. Pat. No. 4,184,146 includes a single pressure-responsive filter switch connected in parallel with an oil temperature-operated switch so that no filter warning signal can be generated when the oil is cold. Also, an RC circuit prevents momentary operation of the filter switch from producing a warning signal. These filter monitoring systems are responsive only to a single level of filter restriction since each system includes only a single filter actuated switch. However, it is well known that the condition of a fluid filter (i.e., its degree of restriction) normally changes only gradually during its service life. Thus, when a filter monitoring system is responsive to only a single level of filter restriction, certain drawbacks exist. For example, such single level systems give no information as to the rate of change of the filter condition, thus giving no indication as to whether the filter restriction is due to normal service buildup or whether it is due to a catastrophic failure of some kind. Furthermore, if the switch actuating level of restriction is set too high, then a situation might occur wherein only a short time interval would elapse between warning signal generation and filter bypassing. If such a situation would occur out in the field where no replacement filter is immediately available, then loss of time and inconvenience would result from shutting the vehicle down while a replacement filter is obtained. Also in this situation, the operator might be tempted to risk serious machine damage by operating the machine after the filter warning is generated when no replacement filter is immediately available. On the other hand, if the switch actuating level of filter restriction is set too low, then the operator might misjudge the seriousness of the condition, thereby delaying filter replacement and risking serious machine damage.